# CO 2 and Long-Term Climate What has moderated Earth surface temperature over the last 4.55 by so that What has moderated Earth surface temperature over.

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CO 2 and Long-Term Climate What has moderated Earth surface temperature over the last 4.55 by so that What has moderated Earth surface temperature over the last 4.55 by so that  All surface vegetation did not spontaneously catch on fire and all lakes and oceans vaporize?  All lakes and ocean did not freeze solid?

Greenhouse Worlds Why is Venus so much hotter than Earth? Why is Venus so much hotter than Earth?  Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbed

Energy Budget Earth’s temperature constant ~15  C Earth’s temperature constant ~15  C  Energy loss must = incoming energy  Earth is constantly receiving heat from Sun, therefore must lose equal amount of heat back to space Heat loss called back radiation Heat loss called back radiation  Wavelengths in the infrared (long-wave radiation) Earth is a radiator of heat Earth is a radiator of heat  If T > 1  K, radiator of heat

Energy Budget Average Earth’s surface temperature ~15  C Average Earth’s surface temperature ~15  C Reasonable assumption Reasonable assumption  Surface of Earth radiates heat with an average temperature of 15  C However, satellite data indicate Earth radiating heat average temperature ~-16  C However, satellite data indicate Earth radiating heat average temperature ~-16  C Why the discrepancy? Why the discrepancy?  What accounts for the 31  C heating?

Energy Budget Greenhouse gases absorb 95% of the long- wave, back radiation emitted from Earth’s surface Greenhouse gases absorb 95% of the long- wave, back radiation emitted from Earth’s surface  Trapped radiation reradiated down to Earth’s surface  Accounts for the 31  C heating  Satellites don’t detect radiation  Muffling effect from greenhouse gases Heat radiated back to space from elevation of about 5 km (top of clouds) average 240 W m -2 Heat radiated back to space from elevation of about 5 km (top of clouds) average 240 W m -2  Keeps Earth’s temperature in balance

Greenhouse Worlds Why is Venus so much hotter than Earth? Why is Venus so much hotter than Earth?  Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbed

What originally controlled C? In solar nebula most carbon was CH 4 In solar nebula most carbon was CH 4  Lost from Earth and Venus  Earth captured 1 in 3000 carbon atoms  Tiny carbon fraction in the atmosphere as CO 2 60 out of every million C atoms60 out of every million C atoms  Bulk of carbon in sediments on Earth CaCO 3 (limestone and dolostone) and organic residues (kerogen)CaCO 3 (limestone and dolostone) and organic residues (kerogen)  Venus probably had similar early planetary history  Most carbon is in atmosphere as CO 2 Venus has conditions that would prevail on Earth Venus has conditions that would prevail on Earth  All CO 2 locked up in sediments were released to the atmosphere

Earth and Venus Water balance different on Earth and Venus Water balance different on Earth and Venus If Venus and Earth started with same components If Venus and Earth started with same components  Venus should have either  Sizable oceans  Atmosphere dominated by steam  H present initially as H 2 O escaped to space  H 2 O transported "top" of the Venusian atmosphere  Disassociated forming H and O atoms  H escaped the atmosphere  Oxygen stirred back to surface Reacted with iron forming iron oxideReacted with iron forming iron oxide

Planetary Evolution Similar Although Earth and Venus started with same components Although Earth and Venus started with same components  Earth evolved such that carbon safely buried in early sediments  Avoiding runaway greenhouse effect Venus built up CO 2 in the atmosphere Venus built up CO 2 in the atmosphere  Build-up led to high temperature  High enough to kill all life If life ever did get a footholdIf life ever did get a foothold  Once hot, could not cool

Why Runaway Greenhouse? Don't know why Venus climate went haywire Don't know why Venus climate went haywire  Extra sunlight Venus receives?  Life perhaps never got started?  No sink for carbon in organic matter  Was the initial component of water smaller than that on Earth?  Did God make Venus as a warning sign?

Early Earth: Faint Young Sun Solar Luminosity 4.55 bya 25% lower than today Solar Luminosity 4.55 bya 25% lower than today Faint young Sun paradox Faint young Sun paradox  If early Earth had no atmosphere or today’s atmosphere  Radiant energy at surface well below 0°C for first 3 billion years of Earth history  No evidence in early Earth rock record that planet was frozen

Early Earth: A Greenhouse World Early Earth was more Venus-like Early Earth was more Venus-like Models indicate that greenhouse required Models indicate that greenhouse required Several greenhouse gases Several greenhouse gases  H 2 O, CO 2, CH 4, NH 3, N 2 O  H 2 O and CO 2 most likely  10 2 -10 3 x PAL CO 2

Early Earth Atmosphere Faint young Sun paradox presents dilemma Faint young Sun paradox presents dilemma  1) What is the source for high levels of greenhouse gases in Earth’s earliest atmosphere?  2) How were those gases removed with time?  Models indicate Sun’s strength increased slowly with time  Geologic record strongly suggests Earth maintained a moderate climate throughout Earth history (i.e., no runaway greenhouse like on Venus)

Source of Greenhouse Gases Input of CO 2 and other greenhouse gases from volcanic emissions Input of CO 2 and other greenhouse gases from volcanic emissions  Most likely cause of high levels in early Earth

Is Volcanic CO 2 Earth’s Thermostat? If volcanic CO 2 emissions provided the early Earth greenhouse, has volcanic activity continuously slowed through geologic time? No, but… If volcanic CO 2 emissions provided the early Earth greenhouse, has volcanic activity continuously slowed through geologic time? No, but… Carbon input balanced by removal Carbon input balanced by removal  Near surface carbon reservoirs Stop all volcanic input of CO 2 Stop all volcanic input of CO 2  Take 270,000 years to deplete atmospheric CO 2  Surface carbon reservoirs (41,700 gt) divided by volcanic carbon input (0.15 gt y -1 )  Rate of volcanic CO 2 emissions have potential to strongly affect atmospheric CO 2 levels on billion- year timescale

Volcanic CO 2 inputs? No geologic, geophysical or geochemical evidence indicates that rates of tectonism decreased slowly through Earth history No geologic, geophysical or geochemical evidence indicates that rates of tectonism decreased slowly through Earth history  Rates of volcanic CO 2 input did not change slowly with time  Volcanic CO 2 emissions did not moderate Earth climate through geologic time  If not inputs, what about a change in removal rate of atmospheric CO 2 ?

Removal of Atmospheric CO 2 Slow chemical weathering of continental rocks balances input of CO 2 to atmosphere Slow chemical weathering of continental rocks balances input of CO 2 to atmosphere Chemical weathering reactions important Chemical weathering reactions important  Hydrolysis and Dissolution

Acid Rain Both natural and anthropogenic processes cause acidification of precipitation. Both natural and anthropogenic processes cause acidification of precipitation. Natural: CO 2 —> H 2 CO 3 (carbonic acid); acids from volcanoes (CO 2 and H 2 S) Natural: CO 2 —> H 2 CO 3 (carbonic acid); acids from volcanoes (CO 2 and H 2 S) Anthropogenic: Oxides of N are by- products (nitric acid) Anthropogenic: Oxides of N are by- products (nitric acid)

Acid Rain Since the 1950s the pH of precipitation in eastern N. America has decreased (i.e., more acidic) significantly over a large area Since the 1950s the pH of precipitation in eastern N. America has decreased (i.e., more acidic) significantly over a large area Other regions effected include eastern Europe and China Other regions effected include eastern Europe and China Consequences of more acidic precipitation include problems for fish and other wildlife and increased chemical weathering, especially of regions with lots of carbonate rocks Consequences of more acidic precipitation include problems for fish and other wildlife and increased chemical weathering, especially of regions with lots of carbonate rocks

Before

After

Hydrolysis Main mechanism of chemical weathering that removes atmospheric CO 2 Main mechanism of chemical weathering that removes atmospheric CO 2 Reaction of silicate minerals with carbonic acid to form clay minerals and dissolved ions Reaction of silicate minerals with carbonic acid to form clay minerals and dissolved ions Summarized by the Urey reaction Summarized by the Urey reaction  CaSiO 3 + H 2 CO 3  CaCO 3 + SiO 2 + H 2 O  Atmospheric CO 2 is carbon source for carbonic acid in groundwater  Urey reaction summarizes atmospheric CO 2 removal and burial in marine sediments  Accounts for 80% of CO 2 removal

Dissolution Kinetics of dissolution reactions faster than hydrolysis Kinetics of dissolution reactions faster than hydrolysis Dissolution reaction neither efficient nor long term Dissolution reaction neither efficient nor long term Dissolution of exposed limestone and dolostone on continents and precipitation of calcareous skeletons in ocean Dissolution of exposed limestone and dolostone on continents and precipitation of calcareous skeletons in ocean  CaCO 3 + H 2 CO 3  CaCO 3 + H 2 O + CO 2  Although no net removal of CO 2  Temporary removal from atmosphere

Atmospheric CO 2 Balance Slow silicate rock weathering balances long- term build-up of atmospheric CO 2 Slow silicate rock weathering balances long- term build-up of atmospheric CO 2  On the 1-100 million-year time scale  Rate of chemical hydrolysis balance rate of volcanic emissions of CO 2  Neither rate was constant with time  Earth’s long term habitably requires only that the two are reasonably well balanced

What Controls Weathering Reactions? Chemical weathering influenced by Chemical weathering influenced by  Temperature  Weathering rates double with 10°C rise  Precipitation  H 2 O is required for hydrolysis Increased rainfall increases soil saturationIncreased rainfall increases soil saturation  H 2 O and CO 2 form carbonic acid  Vegetation  Respiration in soils produces CO 2 CO 2 in soils 100-1000x higher than atmospheric CO 2CO 2 in soils 100-1000x higher than atmospheric CO 2

Climate Controls Chemical Weathering Precipitation closely linked with temperature Precipitation closely linked with temperature  Warm air holds more water than cold air Vegetation closely linked with precipitation and temperature Vegetation closely linked with precipitation and temperature  Plants need water  Rates of photosynthesis correlated with temperature

Chemical Weathering: Earth’s Thermostat? Chemical weathering can provide negative feedback that reduces the intensity of climate warming Chemical weathering can provide negative feedback that reduces the intensity of climate warming

Chemical Weathering: Earth’s Thermostat? Chemical weathering can provide negative feedback that reduces the intensity of climate cooling Chemical weathering can provide negative feedback that reduces the intensity of climate cooling

Greenhouse vs. Faint Young Sun Cold surface temperatures created by the faint young Sun compensated by stronger atmospheric CO 2 greenhouse effect Cold surface temperatures created by the faint young Sun compensated by stronger atmospheric CO 2 greenhouse effect

Volcanism & Weathering Volcanism on early Earth probably produced more atmospheric CO 2 Volcanism on early Earth probably produced more atmospheric CO 2  Counteracted lower radiant energy and warmed our planet  Volcanism did not slow at same rate as Sun increase in strength Early Earth probably still cold Early Earth probably still cold  Weathering slow  Continents small  Continental crustal rocks silica-poor (basaltic)  Stoichiometry of Urey reaction different  Less efficient CO 2 removal from atmosphere

Greenhouse vs. Faint Young Sun When solar luminosity strengthen, chemical weathering increased and helped transfer atmospheric CO 2 into sediments When solar luminosity strengthen, chemical weathering increased and helped transfer atmospheric CO 2 into sediments

Volcanism & Weathering As solar luminosity increased As solar luminosity increased  Earth warmed and became wetter  Chemical weathering increased  CO 2 levels dropped Continental crust began to grow ~2 bya Continental crust began to grow ~2 bya  Became more siliceous (granitic) Slow warming of Earth Slow warming of Earth  Caused changes in chemical weathering  Moderated Earth’s climate

Tectonic Carbon Cycling Carbon cycles continuously between rock reservoir and atmosphere Carbon cycles continuously between rock reservoir and atmosphere  CO 2 removed from atmosphere by chemical weathering, deposited in ocean sediments, subducted and returned by volcanism

Tectonic Carbon Cycling Carbon cycles continuously between rock reservoir and atmosphere Carbon cycles continuously between rock reservoir and atmosphere  Plate tectonic processes dependant on presence of water  Acts as a lubricant for plate subduction

Organic Carbon Burial Affect CO 2 If the rate of organic carbon burial increases, less organic matter available for decomposition and less carbon returned to the atmosphere as CO 2 If the rate of organic carbon burial increases, less organic matter available for decomposition and less carbon returned to the atmosphere as CO 2  Atmospheric CO 2 reservoir shrinks

Organic Carbon Burial Affect O 2 If the rate of organic carbon burial increases, less organic matter available for decomposition and less oxygen is used during decomposition If the rate of organic carbon burial increases, less organic matter available for decomposition and less oxygen is used during decomposition  Atmospheric O 2 reservoir grows

Importance of Solar Irradiance Affect planetary climate development Affect planetary climate development  Earth vs. Venus vs. Mars Faint young Sun paradox Faint young Sun paradox Glacial-interglacial variations in solar irradiance Glacial-interglacial variations in solar irradiance  Milankovitch Cycles Global Dimming Global Dimming  Affect of aerosols on cloud albedo

The Climate System

Global Dimming Describes the gradual reduction in the amount of total solar radiation at the Earth surface since 1950’s Describes the gradual reduction in the amount of total solar radiation at the Earth surface since 1950’s Recognized since 1989 (Ohmura 1989, Russak 1990, Stanhill and Moreshet 1992) Recognized since 1989 (Ohmura 1989, Russak 1990, Stanhill and Moreshet 1992)  2-3% reduction per decade  Largest reduction in northern hemisphere  Not due to reduction in strength of Sun Experiments in the Maldives (INDOEX) Experiments in the Maldives (INDOEX)  Linked global dimming to pollution

Effect of Pollution on Indian Ocean Pollution from India and Asia carried into Indian Ocean by winter monsoon winds Pollution from India and Asia carried into Indian Ocean by winter monsoon winds Brown haze from aerosols surface to 3 km altitude Brown haze from aerosols surface to 3 km altitude  Soot, sulfates, nitrates, organic particles, fly ash and mineral particles Haze particles scatter solar radiation reflecting sunlight Haze particles scatter solar radiation reflecting sunlight  In the polluted INDOEX region, haze particles reduce the solar radiation absorbed the ocean surface by ~10% South of the ITCZ, the sky was clear South of the ITCZ, the sky was clear

INDOEX Airborne particles over the Indian Ocean are different from those over North America and Europe Airborne particles over the Indian Ocean are different from those over North America and Europe  Airborne particles over the northern Indian Ocean are unusually dark  Contain large amounts of soot and other materials from incompletely burned fuels and wastes  Dark aerosols lead to increased absorption of solar radiation  Advanced pollution control technologies remove the dark material and yield particles that are relatively "white" Thus the impact of Asian pollution particles on climate processes appears fundamentally different from that of American and European pollution particles Thus the impact of Asian pollution particles on climate processes appears fundamentally different from that of American and European pollution particles

Effect of Contrails The potential of condensation trails (contrails) from jet aircraft to affect regional-scale surface temperatures has been debated for years Difficult to verify until an opportunity arose as a result of the three-day grounding of all commercial aircraft in the United States in the aftermath of the terrorist attacks on 11 September 2001 An anomalous increase in the average difference between the daytime maximum and night-time minimum temperatures for the period 11–14 September 2001 Persisting contrails can reduce the transfer of both incoming solar and outgoing infrared radiation and so reduce the daily temperature range

Contrails

And now, some good news Global dimming has reversed since 1990 Global dimming has reversed since 1990 Studies of the amount of sunlight making it through the atmosphere suggest that our air is getting cleaner   Reduced industrial emissions and the use of particulate filters ‘Global dimming’ has been noticed since measurements began in the late 1950s, but consensus that it was a global phenomenon was reached only since 2003 Studies published in 2005 indicate dimming replaced by brightening since 1990 Studies published in 2005 indicate dimming replaced by brightening since 1990

Global Dimming & Brightening

From Dimming to Brightening Decline in solar radiation at land surfaces documented in observational records up to 1990   24 stations distributed globally show statistically significant decrease New data from 1990 to the present (mostly from the Northern Hemisphere) show that the dimming did not persist into the 1990s   Data from 300 stations available New data show widespread brightening since the late 1980s  But only up to values found at ~1960

Wild et al. Science 308, 847-850 (2005)

Solar Irradiance 1990- Best of the best sites Best of the best sites  High quality calibration  Long time-series No sites show a decrease in solar irradiance No sites show a decrease in solar irradiance Six sites show increase in solar irradiance Six sites show increase in solar irradiance Matches well global trends showing decrease in cloudiness from 1990- Matches well global trends showing decrease in cloudiness from 1990-  Links effects of pollution with cloud formation High-resolution data from 1990- allows recognition of natural changes High-resolution data from 1990- allows recognition of natural changes  Effects of Pinatubo eruption are clearly evident in records

Implications? Decreasing solar irradiance leads to decreasing surface temperature on Earth Decreasing solar irradiance leads to decreasing surface temperature on Earth Instrumental temperature records show warming trend over last century Instrumental temperature records show warming trend over last century  Attributed to greenhouse effect, mainly rise in atmospheric CO 2 levels Has global dimming masked the effects of global temperature rise due to increased atmospheric CO 2 ? Has global dimming masked the effects of global temperature rise due to increased atmospheric CO 2 ? Will the trend towards cleaner air affect global temperatures by increasing solar irradiance? Will the trend towards cleaner air affect global temperatures by increasing solar irradiance?

Carbon Dioxide

The Earth System

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